LU-1963 ptlrpc: Incorrect accounting for srv_hpreq_ratio

LU-1963 ptlrpc: Incorrect accounting for srv_hpreq_ratio

Allow one thread to always handle HP requests.
Without this patch 1 thread dedicated to serve HP requests
will do nothing if all normal requests are in progress for
a long time but there are HP requests in input queue.

Xyratex-bug-id: MRP-661
Signed-off-by: Andriy Skulysh <Andriy_Skulysh@xyratex.com>
Reviewed-by: Alexey Lyashkov <alexey_lyashkov@xyratex.com>
Reviewed-by: Nathan Rutman <nathan.rutman@xyratex.com>
Change-Id: I35f5329ca019a5d2e2dcee9d7a13eaa74e85233e
Reviewed-on: http://review.whamcloud.com/4008
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Hudson
Reviewed-by: Keith Mannthey <keith.mannthey@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>